The purpose of our proposed work is to clarify speciation in yeast and to determine phylogenetic relationships among yeast species by molecular techniques. The description of new species of yeast is usually based on about 50 morphologic and physiologic criteria. This number of phenotypic traits has proven to be inadequate for the delimitation of species in yeast systematics. Similarly, these traits provide little information on evolutionary relationship between different yeasts. We plan (a) to study strains of species from different but related ecological niches to determine variation in nuclear DNA base composition; (b) DNA/DNA reassociation experiments between nuclear DNAs of what appear to be closely related species with similar mol% (G plus C) and express the results in percent complementarity of heterologous versus homologous DNA; (c) to conduct immunological comparisons of the enzyme superoxide dismutase in species that have low DNA complementarity to establish evolutionary divergence expressed in immunological distances; and (d) to study differences in electrophoretic mobility among a group of enzymes involved in the central metabolism of yeast strains representing species that appear to have progressively diverged from each other.